Evaporator, rankine cycle apparatus, and combined heat and power system

ABSTRACT

An evaporator which heats working fluid with high-temperature fluid to evaporate the working fluid includes: a working fluid channel which is arranged in a flow direction of the high temperature fluid and through which the working fluid flows; and a temperature sensor which is provided for the working fluid channel. A part of the working fluid channel is exposed to outside of a housing of the evaporator, and the temperature sensor is provided in the part of the working fluid channel exposed to the outside of the housing of the evaporator in a region other than an inlet of the working fluid channel into which the working fluid flows from the outside of the evaporator and other than an outlet of the working fluid channel through which the working fluid flows out of the evaporator. The output value of the temperature sensor is used to adjust the temperature of the working fluid in the evaporator.

BACKGROUND

1. Technical Field

The disclosure relates to an evaporator, a Rankine cycle apparatus, anda combined heat and power system.

2. Description of the Related Art

As well known by those skilled in the art, a Rankine cycle is atheoretical cycle with a steam turbine. Research and development on theRankine cycle have been conducted for a long time. On the other hand, asdescribed in US Patent Application Publication No. 2003/0213854(hereinafter referred to as a Patent Literature 1), research anddevelopment have been also carried out for a combined heat and powersystem which employs the Rankine cycle. The combined heat and powersystem (hereinafter referred to a CHP system) is a system whichsimultaneously provides plural forms of energy, such as heat and power,generated from one or plural resources. In recent years, attention hasbeen focused on not only large-scale CHP systems but also CHP systemswhich are installed in relatively small facilities such as hospitals,schools, and libraries and, moreover, focused on household CHP systems(so called micro CHP systems).

SUMMARY

The CHP system of Patent Literature 1 is configured to obtain electricpower by using combustion gas generated by a gas boiler as heat energyfor a Rankine cycle apparatus. Moreover, Patent Literature 1 discloses astructure of an evaporator to prevent gas-phase organic working fluidfrom being excessively heated by the boiler.

The evaporator disclosed in Patent Literature 1 may be effective whenthe Rankine cycle apparatus is operating stably. However, theconfiguration disclosed in Patent Literature 1 is not enough to preventthe working fluid from being heated excessively.

One non-limiting and exemplary embodiment provides a new technique toprevent working fluid from being excessively heated in the evaporator.

In one general aspect, the techniques disclosed here feature anevaporator which heats working fluid with high-temperature fluid toevaporate the working fluid, the evaporator including:

a working fluid channel which is arranged to form a plurality of stagesin a flow direction of the high-temperature fluid and through which theworking fluid flows, wherein

the evaporator further including a first temperature sensor which isprovided for the working fluid channel,

the working fluid channel is arranged to form a meander shape in theplurality of stages, and bent portions of the meander shape are exposedto the outside of a housing of the evaporator,

the plurality of stages include a first stage located most upstream inthe flow direction of the high-temperature fluid and a stage other thanthe first stage,

the working fluid channel allows the working fluid to flow out of theevaporator through an outlet of the working fluid channel which isincluded in the stage other than the first stage,

the first temperature sensor is provided downstream of a particularpoint in the flow direction of the working fluid in a part of theworking fluid channel exposed to the outside of the housing of theevaporator, the particular point being at a distance of L/2 upstream inthe flow direction of the working fluid from the downstream end of thepart of the working fluid channel forming the first stage where L iswhole length of working fluid channel forming the first stage, and

an output value of the first temperature sensor is used to adjusttemperature of the working fluid in the evaporator.

The evaporator described above may prevent working fluid from beingheated excessively.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of a CHP system according toEmbodiment 1 of the disclosure;

FIG. 2A is a configuration diagram of an evaporator of a Rankine cycleapparatus illustrated in FIG. 1;

FIG. 2B is a schematic plan view of the evaporator illustrated in FIG.2A;

FIG. 2C is a schematic plan view of an evaporator according to amodification;

FIG. 3 is a flowchart of a control executed by a control circuit;

FIG. 4 is a diagram showing a relationship between the temperature ofworking fluid and the temperature of inner wall surfaces of heatexchanger tubes in the evaporator;

FIG. 5 is a configuration diagram of a CHP system according toEmbodiment 2 of the disclosure;

FIG. 6A is a configuration diagram of an evaporator of a Rankine cycleapparatus illustrated in FIG. 5;

FIG. 6B is a schematic plan view of the evaporator illustrated in FIG.6A;

FIG. 7 is a flowchart of a control executed by a control circuit;

FIG. 8 is a diagram showing a relationship between the temperature ofworking fluid and the temperature of inner wall surfaces of heatexchanger tubes in the evaporator;

FIG. 9 is a configuration diagram of a CHP system according toEmbodiment 3 of the disclosure;

FIG. 10A is a configuration diagram of an evaporator of a Rankine cycleapparatus illustrated in FIG. 9;

FIG. 10B is a schematic plan view of the evaporator illustrated in FIG.10A;

FIG. 11 is a flowchart of a control executed by a control circuit;

FIG. 12 is a diagram showing a relationship between the temperature ofworking fluid and the temperature of inner wall surfaces of heatexchanger tubes in the evaporator;

FIG. 13 is a configuration diagram of a CHP system according toEmbodiment 4 of the disclosure;

FIG. 14 is a configuration diagram of an evaporator of a Rankine cycleapparatus illustrated in FIG. 13; and

FIG. 15 is a configuration diagram of an evaporator of a conventionalRankine cycle apparatus.

DETAILED DESCRIPTION Underlying Knowledge Forming Basis of the PresentDisclosure

In an evaporator of a Rankine cycle apparatus, excessive heat exchangebetween high-temperature fluid such as combustion gas generated by a gasburner and gas-phase working fluid could cause problems includingthermal decomposition of the working fluid, deterioration of lubricantoil, and the like. Such problems become pronounced when using organicworking fluid or when using an expander requiring lubricant oil.

To avoid the aforementioned problems, Patent Literature 1 proposes anevaporator 104 having a structure illustrated in FIG. 15. The evaporator104 is provided with an inlet 110A for working fluid in the downstreamregion of flow channels 104C for high-temperature fluid (combustion gasgenerated by a burner). The working fluid flowing into a distal side104D through the inlet 110A exchanges heat with the high-temperaturefluid in a countercurrent manner. The working fluid is then fed througha connecting tube 104H to a proximal side 104E, which is located in theupstream side of the flow channels 104C for the high-temperature fluid.In the proximal side 104E, the working fluid flows through first andsecond sections 104E1 and 104E2 in this order. In the proximal side104E, the working fluid thus exchanges heat with the high-temperaturefluid in a parallel-flow manner. An outlet 110C for the working fluid isprovided near the center of an enclosure 104A. The numerals in thecircles (representing heat exchanger tubes) show examples of thetemperature (degree Fahrenheit) of the working fluid.

According to the evaporator 104 illustrated in FIG. 15, it is inferredthat the working fluid can be prevented from being heated excessively.This is because the working fluid is inferred to be in the liquid-phasestate in the distal side 104D, be in the liquid-phase state orgas-liquid two-phase state in the first section 104E1, and be in thegas-liquid two-phase state or gas-phase state in the second section104E2. However, the inference is only possible for the Rankine cycleapparatus being in stable operation. For example, when the circulationflow rate of the working fluid is reduced with a change in electricitydemand, the working fluid can change to the gas-phase state already inthe first section 104E1. In the first section 104E1, the gas-phaseworking fluid could be therefore heated excessively.

Reducing the heat of the burner in accordance with the circulation flowrate of the working fluid is one of the effective means but is notadequate from the viewpoint of responsiveness. In recent years,moreover, attempts to use solid fuel, such as biomass or wood pellets,instead of gas fuel are being studied. The combustion of solid fuel inpellet boilers is not as stable as combustion of gas fuel in gasboilers. Moreover, the pellet boilers are not suitable for quicklyincreasing or reducing the heat. The technique to prevent the workingfluid from being heated excessively is therefore becoming moreimportant. Based on the aforementioned knowledge, the inventor reachesthe invention of each aspect described below.

An evaporator according to a first aspect of the disclosure is theevaporator which heats working fluid with high-temperature fluid toevaporate the working fluid, the evaporator including: a working fluidchannel which is arranged to form a plurality of stages in a flowdirection of the high-temperature fluid and through which the workingfluid flows; and a first temperature sensor which is provided for theworking fluid channel. The working fluid channel is arranged to form ameander shape in the plurality of stages, and bent portions of themeander shape are exposed to the outside of a housing of the evaporator.The plurality of stages include a first stage located most upstream inthe flow direction of the high-temperature fluid and a stage other thanthe first stage. The working fluid channel allows the working fluid toflow out of the evaporator through an outlet of the working fluidchannel which is included in the stage other than the first stage. Thefirst temperature sensor is provided downstream of a particular point inthe flow direction of the working fluid in a part of the working fluidchannel exposed to the outside of the housing of the evaporator, theparticular point being at a distance of L/2 upstream in the flowdirection of the working fluid from the downstream end of the workingfluid channel forming the first stage where L is whole length of thepart of the working fluid channel forming the first stage. The outputvalue of the first temperature sensor is used to adjust temperature ofthe working fluid in the evaporator.

As previously described, working fluid is thermally decomposed at apredetermined temperature or higher. It is therefore necessary to keepthe temperature of the inner wall surface of the working fluid channellower than the predetermined temperature. According to the first aspect,the temperature of the inner wall surface of the working fluid channelin the evaporator can be kept lower than the predetermined temperatureby using the output value of the first temperature sensor to adjust thetemperature of the working fluid in the evaporator. The working fluidcan be therefore prevented from being thermally decomposed. The reasonbecause the evaporator according to the first aspect exerts theaforementioned effect is described in detail below.

In the process of the working fluid channel being heated by thehigh-temperature fluid, how likely the temperature of the inner wallsurface of the working fluid channel is to increase depends on the stateof the working fluid flowing through the working fluid channel.Specifically, the temperature of the inner wall surface of the workingfluid channel is more likely to increase when the working fluid flowingthrough the working fluid channel is in the gas-phase state than in thegas-liquid two-phase state. This is because the inner wall surface ofthe working fluid channel has a lower heat transfer coefficient when theworking fluid flowing through the same working fluid channel is in thegas-phase state than in the gas-liquid two-phase state. When the workingfluid is in the gas-liquid two-phase state, the inner wall surface ofthe working fluid channel has a high enough heat transfer coefficient.Accordingly, heat transfer to the working fluid prevents an increase intemperature of the inner wall surface. On the other hand, when theworking fluid is in the gas-phase state, the inner wall surface of theworking fluid channel has a low heat transfer coefficient. Accordingly,heat transfer to the working fluid is small, and the temperature of theinner wall surface increases.

The evaporator generates working fluid in the gas-phase state fromworking fluid in the partially liquid-phase state. The ratio ofgas-phase working fluid in the working fluid channel therefore increasesdownstream in the flow direction of the working fluid.

The outlet of the working fluid channel which allows the working fluidto flow out of the evaporator is located most downstream in the flowdirection of the working fluid. In an evaporator in which the workingfluid channel is arranged to form the plurality of stages in the flowdirection of the high-temperature fluid, the stage including the outletof the working fluid channel, which allows the working fluid to flow outof the evaporator, is located on the relatively downstream portion inthe flow direction of the working fluid. Accordingly, the stageincluding the outlet of the working fluid channel, which allows theworking fluid to flow out of the evaporator, includes a higher ratio ofgas-phase working fluid in the working fluid channel than the anotherstage.

Moreover, in an evaporator, the high-temperature fluid increases intemperature upstream in the flow direction of the high-temperaturefluid.

Accordingly, in the evaporator in which the working fluid channel isarranged to form the plurality of stages in the flow direction of thehigh-temperature fluid, locating the stage including the outlet of theworking fluid channel, which allows the working fluid to flow out of theevaporator, most upstream in the flowing direction of thehigh-temperature fluid means that the stage including a part of theworking fluid channel which is most likely to increase in temperature isprovided in the hottest place. Accordingly, the temperature of the innerwall surface of the working fluid channel forming the same stage is morelikely to exceed the decomposition temperature of the working fluid,causing thermal decomposition of the working fluid.

On the other hand, the evaporator of the first aspect includes: thefirst stage located most upstream in the flow direction of thehigh-temperature fluid; and the stage other than the first stage. Theworking fluid flows out of the evaporator through the outlet of theworking fluid channel included in the stage other than the first stage.In other words, the first stage is located most upstream in the flowdirection of the high-temperature fluid, but the stage including theoutlet of the working fluid channel, which allows the working fluid toflow out of the evaporator, is not located most upstream. This canreduce the risk that the temperature of the inner wall surface of theworking fluid channel will increase to the decomposition temperature ofthe working fluid or higher.

Moreover, the evaporator of the disclosure includes the firsttemperature sensor, which is provided downstream of a particular pointin the flow direction of the working fluid in a part of the workingfluid channel exposed to the outside of the housing of the evaporator.Herein, the particular point is at a distance of L/2 (L is whole lengthof the part of the working fluid channel forming the first stage)upstream in the flow direction of the working fluid from the downstreamend of the part of the working fluid channel forming the first stage.The working fluid can be therefore prevented from being excessivelyheated and thermally decomposed. The reason therefor is described below.

When the working fluid changes from the gas-liquid two-phase state tothe gas-phase state in the first stage, which is located most upstreamin the flow direction of the high-temperature fluid, the temperature ofthe inner wall surface of the working fluid channel is the highest atthe downstream end of the part of the working fluid channel forming thefirst stage because of the following reason.

As previously described, the temperature of the inner wall surface ofthe working fluid channel is more likely to increase when the workingfluid flowing through the same working fluid channel is in the gas-phasestate than in the gas-liquid two-phase state. When the working fluidchanges from the gas-liquid two-phase state to the gas-phase state inthe first stage, the inner surface of a part of the working fluidchannel through which gas-phase working fluid flows is more likely toincrease in temperature. Moreover, as previously described, thehigh-temperature fluid increases in temperature upstream in the flowdirection of the high-temperature fluid. Since the first stage islocated most upstream in the flow direction of the high-temperaturefluid, the part of the working fluid channel forming the first stage isexposed to higher temperature than the another stage. Furthermore, sincethe working fluid flows through the working fluid channel whileexchanging heat with the high-temperature fluid in the evaporator, theworking fluid increases in temperature toward the downstream portion ofthe working fluid channel. The downstream end of the part of the workingfluid channel forming the first stage therefore has the highesttemperature in the part of the working fluid channel forming the firststage. Accordingly, when the state of the working fluid changes from thegas-liquid two-phase state to the gas-phase state in the first stage,which is located most upstream in the flow direction of thehigh-temperature fluid, the temperature of the inner wall surface of theworking fluid channel is the highest at the downstream end of the partof the working fluid channel forming the first stage.

In the evaporator of the first aspect, the first temperature sensor isprovided downstream of the particular point, which is at a distance ofL/2 (L is whole length of the part of the working fluid channel arrangedin the first stage) upstream in the flow direction of the working fluidfrom the downstream end of the part of the working fluid channel formingthe first stage, in the flow direction of the working fluid in a part ofthe working fluid channel exposed to the outside of the housing of theevaporator. By using the first temperature sensor, the evaporator of thefirst aspect can acquire the temperature of the working fluid at aportion in a range of L/2 upstream in the flow direction of the workingfluid from the downstream end. Accordingly, based on the output value ofthe first temperature sensor, the temperature of the inner wall surfaceof the working fluid channel at the downstream end, which has thehighest temperature in the inner wall surface of the working fluidchannel, can be easily predicted (see FIGS. 4 and 8). The working fluidcan be therefore prevented from being excessively heated and thermallydecomposed by using the output value of the first temperature sensor toadjust the temperature of the working fluid.

In a second aspect, for example, the first temperature sensor of theevaporator according to the first aspect may be provided in a region ofL/2 downstream in the flow direction of the working fluid from thedownstream end of the part of the working fluid channel forming thefirst stage.

According to the second aspect, the place where the first temperaturesensor is provided is within the region of L/2 downstream in the flowdirection of the working fluid from the downstream end of the part ofthe working fluid channel forming the first stage. Accordingly, theplace where the first temperature sensor is provided is limited to thevicinity of the downstream end. This can facilitate prediction of thetemperature of the inner wall surface of the working fluid channel atthe downstream end.

In a third aspect, for example, the plurality of stages of theevaporator according to the first aspect may include a second stagelocated next to the first stage downstream in the flow direction of thehigh-temperature fluid, and the first temperature sensor may be providedbetween the first and second stages.

According to the third aspect, the place where the first temperaturesensor is provided is limited to between the first stage and the secondstage located next to the first stage downstream in the flow directionof the high-temperature fluid. Accordingly, the place where the firsttemperature sensor is provided is limited to an area closer to thedownstream end, thus further facilitating prediction of the temperatureof the inner wall surface of the working fluid channel at the downstreamend.

In a fourth aspect, for example, the plurality of stages of theevaporator according to any one of the first to third aspects mayinclude a third stage which is located most downstream in the flowdirection of the high-temperature fluid. The working fluid channel mayallow the working fluid to enter the evaporator through an inlet of theworking fluid channel which is included in the third stage.

According to the fourth aspect, the working fluid flows into theevaporator through the inlet of the working fluid channel included inthe third stage. The high-temperature fluid and the working fluidflowing through the working fluid channel can thereby exchange heatefficiently.

In a fifth aspect, the working fluid channel of the evaporator accordingto any one of the first to fourth aspects may include a plurality ofheat exchanger tubes provided within the housing of the evaporator and aplurality of connecting tubes corresponding to the bent portions of themeander shape.

In a sixth aspect, at least the part of the working fluid channelforming the first stage in the evaporator according to the first tofifth aspects may be an inner grooved pipe.

According to the sixth aspect, the working fluid swirls in the innergrooved pipe. The swirling flow can reduce a local increase intemperature of the working fluid in the inner grooved pipe.

In a seventh aspect, the evaporator according to the first to sixthaspects may further include a heat insulator surrounding the firsttemperature sensor so that the heat insulator reduces thermal influenceof an environment around the evaporator on the first temperature sensor.

According to the seventh aspect, the temperature of the working fluidcan be accurately detected.

In an eighth aspect, the evaporator according to the first to sixthaspects may further include a partition provided between the firsttemperature sensor and an environment around the evaporator, in whichthe partition reduces thermal influence of the environment around theevaporator on the first temperature sensor.

According to the eighth aspect, the temperature of the working fluid canbe accurately detected.

In a ninth aspect, thermal influence of an environment around theevaporator according to the first to sixth aspects on the firsttemperature sensor may be reduced.

According to the ninth aspect, the temperature of the working fluid canbe accurately detected.

In a tenth aspect, the evaporator according to the first to ninthaspects may be a fin tube heat exchanger.

A Rankine cycle apparatus according to an eleventh aspect includes: apump which pressurizes the working fluid; the evaporator according toany one of the first to tenth aspects which receives the working fluiddischarged from the pump; an expander which expands the working fluidheated by the evaporator; a condenser which cools the working fluiddischarged from the expander; and a control circuit.

According to the eleventh aspect, the same effect as that of the firstaspect can be obtained.

In a twelfth aspect, for example, the control circuit of the Rankinecycle apparatus according to the eleventh aspect may reduce supply ofthe high-temperature fluid when the temperature acquired by the firsttemperature sensor in the evaporator is not lower than a predeterminedvalue and increase the supply of the high-temperature fluid when thetemperature acquired by the first temperature sensor in the evaporatoris lower than the predetermined value.

In a thirteenth aspect, for example, the control circuit of the Rankinecycle apparatus according to the eleventh aspect may increase rotationspeed of the pump when the temperature acquired by the first temperaturesensor in the evaporator is not lower than a predetermined value andreduce the rotation speed of the pump when the temperature acquired bythe first temperature sensor in the evaporator is lower than thepredetermined value.

In a fourteenth aspect, for example, the control circuit of the Rankinecycle apparatus according to the eleventh aspect may increase rotationspeed of the expander when the temperature acquired by the firsttemperature sensor in the evaporator is not lower than a predeterminedvalue and reduce the rotation speed of the expander when the temperatureacquired by the first temperature sensor in the evaporator is lower thanthe predetermined value.

In a fifteenth aspect, for example, the Rankine cycle apparatusaccording to the eleventh aspect may further include a control valvecapable of controlling a circulation flow rate of the working fluid, inwhich the control circuit increases an opening of the control valve whenthe temperature acquired by the first temperature sensor in theevaporator is not lower than a predetermined value and reduces theopening of the control valve when the temperature acquired by the firsttemperature sensor in the evaporator is lower than the predeterminedvalue.

In a sixteenth aspect, the control circuit of the Rankine cycleapparatus according to the eleventh aspect may reduce temperature of thehigh-temperature fluid when the temperature acquired by the firsttemperature sensor in the evaporator is not lower than a predeterminedvalue and increase the temperature of the high-temperature fluid whenthe temperature acquired by the first temperature sensor in theevaporator is lower than the predetermined value.

In another aspect, for example, the Rankine cycle apparatus according tothe eleventh aspect may further include a fan which is provided withinthe evaporator and supplies air to the high-temperature fluid. Thecontrol circuit may increase the rotation speed of the fan to lower thetemperature of the high-temperature fluid when the temperature acquiredby the first temperature sensor in the evaporator is not lower than apredetermined value and reduce the rotation speed of the fan to increasethe temperature of the high-temperature fluid when the temperatureacquired by the first temperature sensor in the evaporator is lower thanthe predetermined value.

According to the twelfth to sixteenth aspects, the working fluid can beprevented from being excessively heated.

In a seventeenth aspect, for example, the evaporator according to anyone of the first to third aspects may further include a secondtemperature sensor which is different from the first temperature sensorand is provided upstream of the first stage in a part of the workingfluid channel exposed to the outside of the housing the evaporator. Theoutput values of the first and second temperature sensors can be used toadjust the temperature of the working fluid in the evaporator.

According to the seventeenth aspect, the two temperature sensors areused, thus implementing more accurate temperature control. Moreover, thestate of the working fluid in the working fluid channel (themost-upstream portion of the working fluid channel in particular) can beaccurately known based on the difference between the temperaturesdetected by the first and second temperature sensors.

In an eighteenth aspect, for example, the temperature of thehigh-temperature fluid of the Rankine cycle apparatus according to anyone of the eleventh to sixteenth aspects may be higher thandecomposition temperature of the working fluid.

The higher the high-temperature fluid, the higher the operationefficiency of the Rankine cycle apparatus.

In a nineteenth aspect, for example, the working fluid of the Rankinecycle apparatus according to any one of the eleventh to sixteenth andeighteenth aspects may be organic working fluid.

According to the nineteenth aspect of the disclosure, by employing theorganic working fluid, the Rankine cycle apparatus can be configured byusing a comparatively low-temperature heat source as well as thehigh-temperature heat source, such as a boiler.

A CHP system according to a twentieth aspect includes: the Rankine cycleapparatus according to any one of the eleventh to sixteenth, eighteenth,and nineteenth aspects; and a heat medium circuit in which a heat mediumflows as a low-temperature heat source cooling the working fluid in thecondenser of the Rankine cycle apparatus.

An evaporator according to a twenty-first aspect is the evaporator whichheats working fluid with high-temperature fluid to evaporate the workingfluid, the evaporator including: a working fluid channel which isarranged in the flow direction of the high temperature fluid and throughwhich the working fluid flows; and a temperature sensor provided for theworking fluid channel. A part of the working fluid channel is exposed tothe outside of a housing of the evaporator. The temperature sensor isprovided in the part of the working fluid channel exposed to the outsideof the housing of the evaporator in a region other than an inlet of theworking fluid channel into which the working fluid flows from theoutside of the evaporator and other than an outlet of the working fluidchannel through which the working fluid flows out of the evaporator, andan output value of the temperature sensor is used to adjust thetemperature of the working fluid in the evaporator.

According to the twenty-first aspect, the temperature sensor is providedin the part of the working fluid channel exposed to the outside of thehousing of the evaporator in a region other than the inlet of theworking fluid channel into which the working fluid flows from theoutside of the evaporator and other than the outlet of the working fluidchannel through which the working fluid flows out of the evaporator. Thetemperature of the working fluid within the heat exchanger can be thusknown, facilitating adjustment of the temperature of the working fluidin the evaporator.

In a twenty-second aspect, for example, in the evaporator of thetwenty-first aspect, the working fluid channel may be arranged to form aplurality of stages in the flow direction of the high-temperature fluidand may be arranged to form a meander shape in the plurality of stages.The part of the working fluid channel exposed to the outside of thehousing of the evaporator may be a bent portion of the meander shape.

In a twenty-third aspect, for example, the plurality of stages of theevaporator according to the twenty-second aspect may include a firststage located most upstream in the flow direction of thehigh-temperature fluid, a third stage located most downstream in theflow direction of the high-temperature fluid, and a second stage betweenthe first and third stages, and the working fluid channel allows theworking fluid to flow from the outside of the evaporator into a part ofthe working fluid channel included in the first stage, to go through thethird stage, and to flow out of the evaporator through a part of theworking fluid channel included in the second stage. The temperaturesensor is provided for a region of the working fluid channel where theworking fluid moves from the third stage to the second stage.

According to the twenty-third aspect, the working fluid channel allowsthe working fluid to flow from the outside of the evaporator to thefirst stage, which is located most upstream in the flow direction of thehigh-temperature fluid among the plurality of stages. As previouslydescribed, in the evaporator, the high-temperature fluid increases intemperature upstream in the flow direction of the high-temperaturefluid. Moreover, since the evaporator generates working fluid in thegas-phase state from working fluid partially in the liquid-phase state,the ratio of gas-phase working fluid in the working fluid channeltherefore increases downstream in the flow direction of the workingfluid. According to the twenty-third aspect, therefore, the stageincluding a part of the working fluid channel which is the least likelyto increase in temperature is provided at the hottest place. It istherefore possible to prevent the temperature of the inner wall surfaceof the part of the working fluid channel forming the same stage fromincreasing to the decomposition temperature of the working fluid orhigher.

In a twenty-fourth aspect, the evaporator according to the twenty-firstaspect may further include a combustor which has a cylindrical shape andwhich generates high-temperature fluid and feeds the high-temperaturefluid radially from the central axis of the cylindrical shape. Theworking fluid channel has a coil shape and is provided around thecombustor.

According to the twenty-fourth aspect, the working fluid channel isprovided around the combustor. In other words, the combustor is providedwithin the coil shape formed by the working fluid channel (in a regionforming the central axis of the coil). Accordingly, the evaporator ofthe twenty-fourth aspect as a whole can be made smaller than theevaporators of the other aspects.

In a twenty-fifth aspect, in a cross-sectional view of the evaporator(when seen in the direction vertical to the central axis of the coilshape), the working fluid channel of the evaporator of the twenty-fourthaspect may include a first section (25 a) which overlaps the evaporatorand a second section (25 b) which is located downstream of the firstsection in the flow direction of the working fluid and does not overlapthe evaporator, and the temperature sensor may be provided between thefirst and second sections in the part of the working fluid channelexposed to the outside of the housing of the evaporator.

According to the twenty-fifth aspect, in the first section as a regionclose to the combustor, the working fluid channel is heated by theheat-temperature fluid at high temperature. Accordingly, from theviewpoint of preventing thermal decomposition of the working fluid, itis suitable that the working fluid is controlled so that in the firstsection, the working fluid changes to the gas-liquid two-phase state orlow-temperature gas-phase state while the working fluid changes to thehigh-temperature gas-phase state in the second section which is distantfrom the combustor. According to the twenty-fifth aspect, the controlcan be easily performed by providing the temperature sensor between thefirst and second sections at the portion exposed to the outside of thehousing of the evaporator.

In a twenty-sixth aspect, the evaporator according to the twenty-fourthor the twenty-fifth aspect may be a coil heat exchanger.

In a twenty-seventh aspect, the evaporator according to the twenty-fifthaspect may further include a structure which is provided between aninlet for receiving the high-temperature fluid and an outlet fordischarging the high-temperature fluid and interrupts the flow of thehigh-temperature fluid to change the flow direction of thehigh-temperature fluid.

According to the twenty-seventh aspect, the flow direction of thehigh-temperature fluid can be determined so as to implement efficientheat exchange.

Hereinafter, a description is given of embodiments of the disclosurewith reference to the drawings. The disclosure is not limited to thefollowing embodiments.

Embodiment 1

As illustrated in FIG. 1, a combined heat and power system 100(hereinafter referred to as a CHP system 100) of Embodiment 1 includes aboiler 10, a Rankine cycle apparatus 20, a heat medium circuit 30, and acontrol circuit 50. The CHP system 100 is capable of providing hot waterand electrical power simultaneously or independently by using heatenergy generated by the boiler 10. Here, the term “simultaneously” meansthat the CHP system 100 is capable of supplying hot water whilesupplying electric power.

The boiler 10 includes a combustion chamber 12 and a combustor 14. Inupper part of the combustion chamber 12, an exhaust port is provided.The combustor 14 is a heat source to generate combustion gas G and islocated within the combustion chamber 12. The combustion gas G generatedin the combustor 14 moves upward in the internal space of the combustionchamber 12 and is discharged to the outside through the exhaust port.When the combustor 14, which generates the combustion gas G, is used asthe heat source of the CHP system 100, high-temperature heat energy canbe easily obtained. This can increase the power generation efficiency ofthe Rankine cycle apparatus 20. Within the boiler 10, another devicesuch as a fan may be provided.

The boiler 10 is a gas boiler, for example. When the boiler 10 is a gasboiler, the combustor 14 is supplied with gas fuel, such as natural gasor biogas. The combustor 14 burns the gas fuel to generate thecombustion gas G at high temperature. The boiler 10 may be anotherboiler such as a pellet boiler. In this case, the combustor 14 issupplied with solid fuel such as wood pellets.

The Rankine cycle apparatus 20 includes an expander 21, a condenser 22,a pump 23, and an evaporator 24. These components are connected into aring in the aforementioned order with plural tubes so as to form aclosed loop. The Rankine cycle apparatus 20 may include a publicly-knownreproducer and the like.

The expander 21 expands the working fluid to convert the expansionenergy of the working fluid to rotation power. The rotary shaft of theexpander 21 is connected to a generator 26. The expander 21 drives thegenerator 26. The expander 21 is a positive displacement expander or aturboexpander, for example. The positive displacement expander is ascroll expander, a rotary expander, a screw expander, a reciprocatingexpander, or the like. The turboexpander is a so-called expansionturbine.

As the expander 21, a positive displacement expander is recommended.Generally, positive displacement expanders exert high expanderefficiency in a wider range of rotation speed than that ofturboexpanders. For example, the positive displacement expanders canoperate at a rotation speed lower than half the rated speed whileretaining the high efficiency. In other words, the power generation bythe positive displacement expanders can be reduced to half the ratedpower generation amount or less with the efficiency maintained high.With the positive displacement expanders, which have suchcharacteristics, it is possible to flexibly respond to changes in powergeneration due to changes in thermal demand. Moreover, it is possible toincrease or reduce the power generation in response to varyingelectricity demand with the efficiency maintained high.

The condenser 22 causes water in the heat medium circuit 30 to exchangeheat with the working fluid discharged from the expander 21, thuscooling the working fluid and heating the water. As the condenser 22, apublicly-known heat exchanger such as a plate-type heat exchanger or adouble-pipe heat exchanger can be used. The type of the condenser 22 isproperly selected depending on the type of the heat medium in the heatmedium circuit 30. When the heat medium in the heat medium circuit 30 isliquid such as water, the condenser 22 can suitably employ a plate-typeheat exchanger or a double-pipe heat exchanger. When the heat medium inthe heat medium circuit 30 is gas such as air, the condenser 22 cansuitably employ a fin tube heat exchanger.

The pump 23 sucks and pressurizes the working fluid flowing out from thecondenser 22 and supplies the pressurized working fluid to theevaporator 24. The pump 23 can be a general positive displacement pumpor turbopump. The general positive displacement pump is a piston pump, agear pump, a vane pump, a rotary pump, or the like. The turbopump is acentrifugal pump, a mixed flow pump, an axial pump, or the like.

The evaporator 24 is a heat exchanger which absorbs heat energy of thecombustion gas G generated by the boiler 10. The evaporator 24 is a fintube heat exchanger, for example. The evaporator 24 is provided withinthe boiler 10 so as to be situated in the channel of the combustion gasG. In Embodiment 1, the evaporator 24 is located right above thecombustor 14. In the evaporator 24, the combustion gas G generated inthe boiler 10 exchanges heat with the working fluid of the Rankine cycleapparatus 20. The working fluid is thereby heated and evaporated.

As the working fluid of the Rankine cycle apparatus 20, organic workingfluid can be preferably used. The organic working fluid is halogenatedhydrocarbon, hydrocarbon, alcohol, or the like. The halogenatedhydrocarbon is R-123, R-245fa, R-1234ze, or the like. The hydrocarbon isan alkane such as propane, butane, pentane, or isopentane. The alcoholis ethanol or the like. The working fluid may be composed of one or amixture of two or more types of those organic working fluids. Theworking fluid can be inorganic working fluid such as water, carbondioxide, or ammonium.

The heat medium circuit 30 is a circuit in which water (the heat medium)as a low-temperature heat source flows, the low-temperature heat sourcecooling the working fluid of the Rankine cycle apparatus 20 in thecondenser 22. The heat medium circuit 30 is connected to the condenser22. The water in the heat medium circuit 30 is heated by the workingfluid discharged from the expander 21. The heat medium circuit 30 isprovided with a pump 31 and a radiator 32. The radiator 32 is a part ofa floor heating appliance in the room, for example. The hot watergenerated by the condenser 22 is supplied to the radiator 32 by the pump31 to be used for room heating. In other words, the heat medium circuit30 is a hot water heating circuit in Embodiment 1. In the case ofheating city water in the condenser 22, the hot water generated by thecondenser 22 can be used for hot water supply. The effective use oflow-temperature waste heat of the working fluid can increase the totalthermal efficiency of the Rankine cycle apparatus 20.

When the heat medium to be heated through the heat medium circuit 30 isliquid such as water or brine like Embodiment 1, the heat medium circuit30 can be composed of plural tubes. On the other hand, when the heatmedium to be heated through the heat medium circuit 30 is gas such asair, the heat medium circuit 30 can be composed of an air trunk or aduct for the gas to flow through. The hot air generated by the condenser22 is supplied into the room to be used for room heating.

The hot water generated by the heat medium circuit 30 can be supplied toother facilities such as a shower, taps, and a hot-water tank. The heatmedium circuit 30 may be configured to be used for re-heatinglow-temperature hot water or to be used to heat city water. The CHPsystem 100 may be configured to supply only electric power by suspendingsupply of hot water through the heat medium circuit 30.

The control circuit 50 controls control objects including the pumps 23and 31, combustor 14, and generator 26. The control circuit 50 needs tohave a control function and includes an arithmetic processing unit (notillustrated) and a storage (not illustrated) storing a control program.Examples of the arithmetic processing unit are a MPU and a CPU. Thestorage is a memory, for example. The control circuit 50 may be a singlecontrol circuit performing centralized control or may be composed ofplural control circuits cooperating with each other for distributedcontrol (the same applies for control circuits of the other embodimentsand modifications). The control circuit 50 stores a program toappropriately operate the CHP system 100.

Next, the structure of the evaporator 24 is described in detail.

As illustrated in FIGS. 2A and 2B, the evaporator 24 includes a workingfluid channel 25 through which the working fluid flows. The workingfluid channel 25 forms plural stages in the flow direction of thecombustion gas G which exchanges heat with the working fluid. Theworking fluid channel 25 is provided with a temperature sensor 35. Basedon output values (detected values) of the temperature sensor 35, thetemperature of the working fluid in the evaporator 24 is adjusted.

In Embodiment 1, the evaporator 24 is a fin tube heat exchangerincluding plural fins 27, plural heat exchanger tubes 28, and pluralconnecting tubes 29. The plural fins 27 are arranged side by side in thehorizontal direction so that the front and back surfaces thereof arepositioned in parallel to the vertical direction. The spaces formedbetween the fins 27 adjacent to each other form an exhaust path of thecombustion gas G. The plural heat exchanger tubes 28 are arranged inplural stages in the flow direction (height direction) of the combustiongas G which exchanges heat with the working fluid. In Embodiment 1, theplural heat exchanger tubes 28 are arranged in three stages in theheight direction. The plural connecting tubes 29 connect the plural heatexchanger tubes 28 to each other. The connecting tubes 29 are so-calledbend tubes.

The working fluid channel 25 is composed of the plural heat exchangertubes 28 and the plural connecting tubes 29. The heat exchanger tubes 28are located within the combustion chamber 12, and the connecting tubes29 are located outside of the combustion chamber 12. The heat exchangertubes 28 are situated in the flow path of the combustion gas G andexchange heat with combustion gas G. The connecting tubes 29 are locatedoutside of the flow path of the combustion gas G and does not directlyexchange heat with the combustion gas G.

Each stage includes plural heat exchanger tubes 28 (four tubes inEmbodiment 1) arranged in the horizontal direction (a direction verticalto the flow direction of the combustion gas G). The heat exchanger tubes28 are therefore arranged in the height direction (the direction Y) andthe horizontal direction (the direction X) in a matrix. The workingfluid flows through the plural heat exchanger tubes 28 located in thesame stage and then fed to the heat exchanger tubes 28 located inanother stage. As illustrated in FIG. 2B, when the evaporator 24 isobserved in the direction vertical to the surfaces of the fins 27, theplural heat exchanger tubes 28 are provided in a staggered arrangement.The plural heat exchanger tubes 28 are connected to each other with theconnecting tubes 29, which are provided at both ends of each heatexchanger tubes 28, so as to form a single flow channel. However, it isunnecessary to form a single channel with all of the heat exchangertubes 28. The heat exchanger tubes 28 may be configured to form two ormore flow channels by using publicly known components such as a divider.Moreover, the heat exchanger tubes 28 and connecting tubes 29 can becomposed of so-called hairpin tubes. In this case, the combination oftwo of the linear heat exchanger tubes 28 and one of the connectingtubes 29 can be replaced with one hairpin tube.

The working fluid channel 25 includes a most-upstream working fluidchannel 25 a, a most-downstream working fluid channel 25 b, and at leastan intermediate working fluid channel 25 c. The most-upstream workingfluid channel 25 a is a flow channel located in the most upstream stagein the flow direction of the combustion gas G. The most-downstreamworking fluid channel 25 a is a channel located in the most downstreamstage in the flow direction of the combustion gas G. The intermediateworking fluid channel 25 c is a flow channel located between themost-upstream and most-downstream working fluid channels 25 a and 25 bin the flow direction of the combustion gas G. The at least anintermediate working fluid channel 25 c may include plural intermediateworking fluid channels 25 c. The most-upstream, most-downstream, andintermediate working fluid channels 25 a, 25 b, and 25 c are eachcomposed of plural heat exchanger tubes 28 and plural connecting tubes29.

One of the heat exchanger tubes 28 constituting the most-downstreamworking fluid channel 25 b serves as an inlet of the evaporator 24 sothat the working fluid entering the evaporator 24 flows through the heatexchanger tubes 28 at first. One of the exchanger tubes 28 constitutingthe intermediate working fluid channel 25 c serves as an outlet of theevaporator 24. The most-upstream working fluid channel 25 a constitutesmiddle part of the working fluid channel 25. The working fluiddischarged from the pump 23 flows through the most-downstream workingfluid channel 25 b, the most-upstream working fluid channel 25 a, andthe intermediate working fluid channel 25 c in this order.

Just after being discharged from the pump 23 and flowing into theevaporator 24, the working fluid is in the liquid-phase state orgas-liquid two-phase state and has the lowest temperature in theevaporator 24. The working fluid flows in the evaporator 24 and isheated by the combustion gas G to be evaporated. At the outlet of theevaporator 24, the working fluid is in the gas-phase state and has thehighest temperature in the evaporator 24. If excessive heat exchangebetween the working fluid and the combustion gas G occurs in theevaporator 24, an excessive increase in temperature of the working fluidcould cause troubles including thermal decomposition of the workingfluid, deterioration of lubricant oil, and the like.

In Embodiment 1, in terms of the flow direction of the working fluid,the temperature sensor 35 is provided downstream of a downstream end 25p of the most-upstream working fluid channel 25 a in the working fluidchannel 25. Based on the output value of the temperature sensor 35, thetemperature of the working fluid in the evaporator 24 is adjusted. To bespecific, control is made so that the temperature of the working fluidat the position of the temperature sensor 35 is maintained at atemperature lower than a setting upper limit temperature. This canprevent the working fluid from being excessively heated. The reasonthereof is as follows. The inner wall surface of the heat exchanger tube28 located at the downstream end 25 p of the most-upstream working fluidchannel 25 a has the highest temperature among the heat exchanger tubes28. It is therefore possible to determine that the working fluid isprevented from being excessively heated when the working fluid has atemperature lower than a setting upper limit temperature at an arbitraryposition downstream of the downstream end 25 p.

In Embodiment 1, the temperature sensor 35 is attached to one of theconnecting tubes 29 constituting the working fluid channel 25 downstreamof the downstream end 25 p of the most-upstream working fluid channel 25a in terms of the flow direction of the working fluid. Specifically, thetemperature sensor 35 is attached to the connecting tube 29 connected tothe downstream end 25 p of the most-upstream working fluid channel 25 a.When the temperature sensor 35 is provided at such a position, theaforementioned effect can be obtained.

The temperature sensor 35 is bonded to the outer circumferential surfaceof the connecting tube 29, for example. A sensing portion (athermocouple, a thermistor, or the like) of the temperature sensor 35may be inserted into the connecting tube 29. Moreover, the temperaturesensor 35 is preferably provided at the position equidistant from eachend of the connecting tube 29. By locating the temperature sensor 35properly away from the heat exchanger tubes 28, the temperature sensor35 can be prevented from being influenced by heat conduction. It istherefore possible to accurately detect the temperature of the workingfluid flowing through the connecting tube 29.

In Embodiment 1, the thermal influence caused by an environment aroundthe evaporator 24 on the temperature sensor 35 is controlled.Specifically, the evaporator 24 further includes a heat insulator 41surrounding the temperature sensor 35. This heat insulator 41 controlsthe thermal influence caused by an environment around the evaporator 24on the temperature sensor 35. The heat insulator 41 surrounds thetemperature sensor 35 and connecting tube 29 to which the temperaturesensor 35 is attached. The connecting tube 29 and heat insulator 41constitute a thermally-insulated connecting tube 29 a. With such aconfiguration, the temperature of the working fluid flowing through theconnecting tube 29 is detected accurately. The heat insulator 41 is madeof woven fabric, non-woven fabric, felt, rock wool, glass wool, siliconesponge, or the like.

In Embodiment 1, the most-upstream working fluid channel 25 a iscomposed of at least an inner grooved pipe. To be specific, at least oneof the heat exchanger tubes 28 constituting the most-upstream workingfluid channel 25 a is composed of an inner grooved pipe. Every heatexchanger tube 28 constituting the most-upstream working fluid channel25 a may be an inner grooved pipe. The plural connecting tubes 29constituting the most-upstream working fluid channel 25 a may be eitherinner grooved pipes or inner smooth pipes. The inner grooved pipes areheat exchanger tubes on the grounds that the working fluid such as acoolant is in the gas-liquid two-phase state and the tubes are appliedto the evaporator. When the heat exchanger tubes 28 have grooves in theinner circumferential surfaces, the working fluid in the liquid-phasestate flows along the grooves to form swirling flow.

In the most-upstream working fluid channel 25 a, because of the frameradiation, each heat exchanger tube 28 receives a relatively largeamount of heat in a particular section (a section facing the frame) ofthe surface thereof and receives a relatively small amount of heat inthe other section. Accordingly, there is a risk that the temperature ofthe heat exchanger tube 28 will increase locally. However, when the heatexchanger tube 28 is an inner grooved pipe, the working fluid swirlswithin the heat exchanger tube 28. The generation of swirling flow canprevent the temperature of the working fluid from locally increasingwithin the heat exchanger tube 28, reducing the local difference intemperature. According to Embodiment 1, it is therefore possible toreduce the influence of local changes in temperature in themost-upstream working fluid channel 25 a and accurately detect thetemperature of the working fluid with the temperature sensor 35.

A part or all of the working fluid channel 25 can be inner grooved pipesin the region downstream of the upstream end 25 q of the most-upstreamworking fluid channel 25 a. The connecting tube 29 connecting themost-upstream working fluid channel 25 a and intermediate working fluidchannel 25 c may be an inner grooved pipe, for example. In this case,the measurement error of the temperature sensor can be reduced.Moreover, the plural heat exchanger tubes 28 and plural connecting tubes29 constituting the intermediate working fluid channel 25 c may be innergrooved pipes. The plural heat exchanger tubes 28 and plural connectingtubes 29 constituting the most-downstream working fluid channel 25 b maybe inner smooth pipes.

Next, with reference to the flowchart of FIG. 3, a description is givenof a control which is executed by the control circuit 50 to adjust thetemperature of the working fluid in the evaporator 24. By executing thecontrol illustrated in FIG. 3, the temperature of the working fluid at aparticular position of the evaporator 24 approximates to the targettemperature. The aforementioned particular position is the positionwhere the temperature sensor 35 is provided, for example. The controlshown in the flowchart of FIG. 3 is started at the same time when theRankine cycle apparatus 20 is activated, for example.

First, temperature Th of the working fluid is detected with thetemperature sensor 35 (step S1). Next, based on the detected temperatureTh, it is determined whether the temperature of the working fluid in theevaporator 24 is excessively high (step S2). Specifically, it isdetermined whether the detected temperature Th is equal to or higherthan the upper limit temperature previously set. When the temperature ofthe working fluid is excessively high, a process to reduce thetemperature of the working fluid is executed. Before execution of theprocess to reduce the temperature of the working fluid, the rotationspeed fp (operation frequency) of the pump 23 is detected (step S3).

Next, the temperature of the working fluid is reduced by reducing thesupply flow rate of the combustion gas G to the evaporator 24. In otherwords, the combustor 14 is controlled to reduce the amount of thecombustion gas G generated per unit time (an amount of combustion heatgenerated per unit time) (step S4). The way of reducing the supply flowrate of the combustion gas G includes reducing fuel supply to thecombustor 14 (the amount of fuel supplied per unit time). When a fan tofeed air into the boiler 10 is provided, the supply flow rate of thecombustion gas G can be reduced by reducing the air flow from the fan.The temperature of the working fluid in the evaporator 24 can be thusadjusted by controlling the supply flow rate of the combustion gas G.

Next, it is determined whether the rotation speed fp of the pump 23 isequal to or lower than the upper limit (step S5). When the rotationspeed fp of the pump 23 is equal to or lower than the upper limit, therotation speed fp of the pump 23 is increased (step S6). The circulationflow rate of the working fluid in the Rankine cycle apparatus 20 therebyincreases. When the rotation speed fp of the pump 23 is increased, theflow rate of the working fluid in the evaporator 24 increases, and thetemperature of the working fluid can decrease. Accordingly, bycontrolling the rotation speed of the pump 23, the temperature of theworking fluid in the evaporator can be adjusted. After the rotationspeed fp of the pump 23 is increased, the temperature Th of the workingfluid is detected with the temperature sensor 35 (step S7). It is thendetermined whether the detected temperature Th is lower than the upperlimit temperature (step S8). The processes of the steps S3 to S8 arerepeated until the temperature Th of the working fluid becomes lowerthan the upper limit temperature.

FIG. 4 illustrates a relationship between the temperature of the workingfluid in the evaporator 24 and the temperature of the inner wallsurfaces of the heat exchanger tubes. The horizontal axis of FIG. 4shows the distance from the inlet of the evaporator 24. The verticalaxis of FIG. 4 shows temperature. In the example shown in FIG. 4, theRankine cycle apparatus 20 is provided with a reheat unit, and theworking fluid is in the gas-liquid two-phase state at the inlet of theevaporator 24.

The working fluid is heated by the combustion gas G to change to thegas-phase state between the inlet and outlet of the evaporator 24. Theworking fluid is in the gas-phase state when flowing out of theevaporator 24. The temperature of the working fluid is constant when theworking fluid is in the gas-liquid two-phase state and increases afterthe working fluid changes to the gas-phase state. The temperature of theinner wall surfaces of the heat exchanger tubes 28 never increasesrapidly as long as the working fluid is in the gas-liquid two-phasestate. Accordingly, the temperature of the working fluid is less likelyto excessively increase in the most-upstream working fluid channel 25 a.When the working fluid is in the gas-liquid two-phase state, the innerwall surfaces of the heat exchanger tubes 28 have a high enough heattransfer coefficient. Accordingly, heat transfer to the working fluidcan prevent an increase in temperature of the inner wall surfaces.

When the working fluid changes from the gas-liquid two-phase state tothe gas-phase state, the temperature of the inner wall surfaces of theheat exchanger tubes 28 rapidly increases. When the working fluid is inthe gas-phase state in the heat exchanger tubes 28, the inner wallsurface of each heat exchanger tube 28 has a low heat transfercoefficient and therefore increases in temperature. When the workingfluid moves from one stage to another in the evaporator 24, thetemperature of the inner wall surfaces of the heat exchanger tubes 28 towhich the working fluid is exposed changes because of a change intemperature of the combustion gas G. The temperature of the combustiongas G is relatively high in the upstream (the first stage) and isrelatively low in the downstream (the second stage). When the workingfluid moves from the first stage to the second stage, the temperature ofthe inner surfaces of the heat exchanger tubes 28 to which the workingfluid is exposed decreases. When the working fluid is in the gas-phasestate, the temperature of the inner wall surfaces of the heat exchangertubes 28 continues increasing along with an increase in temperature ofthe working fluid. Accordingly, the temperature of the working fluid isthe highest at the outlet of the evaporator 24. Accordingly, the outletof the evaporator 24 is preferably provided at the downstream portion inthe flow direction of the combustion gas G.

When the outlet of the evaporator 24 is provided at the downstreamportion in the flow direction of the combustion gas G, the inner wallsurface of the heat exchanger tube 28 located at the downstream end 25 pof the most-upstream working fluid channel 25 a has the highesttemperature among the heat exchanger tubes 28 constituting the workingfluid channel 25. In other words, the working fluid is most likely to bethermally decomposed at the downstream end 25 p. Accordingly, it issignificant from the viewpoint of safety to know the temperature of theheat exchanger tube 28 constituting the downstream end 25 p of themost-upstream working fluid channel 25 a and control the Rankine cycleapparatus 20 so that the temperature of the working fluid is lower thanthe decomposition temperature.

In the example shown in FIG. 4, the working fluid changes from thegas-liquid two-phase state to the gas-phase state in the most-upstreamworking fluid channel 25 a. From the viewpoint of safety, it isdesirable that the working fluid is in the gas-liquid two-phase stateall through the most-upstream working fluid channel 25 a. However, theoperation conditions of the Rankine cycle apparatus 20 vary from seasonto season or the like. When the working fluid is in the gas-liquidtwo-phase state all through the most-upstream working fluid channel 25a, therefore, there is a possibility that the working fluid in thegas-phase state at a target temperature cannot be supplied to theexpander 21 in a particular operation condition. Accordingly, it is notalways inhibited that the working fluid changes from the gas-liquidtwo-phase state to the gas-phase state in the most-upstream workingfluid channel 25 a. If the temperature of the working fluid is lowerthan the upper limit temperature at the position where the temperaturesensor 35 is provided, the safety is ensured.

The technique disclosed in the specification is effective especiallywhen the working fluid is an organic working fluid. To be more specific,the technique disclosed in the specification is effective especiallywhen the temperature of the combustion gas G is higher than thedecomposition temperature of the working fluid. If organic working fluidis employed, the Rankine cycle apparatus can be configured by using acomparatively low temperature heat source as well as a high temperatureheat source, such as the boiler 10. The higher the temperature of thecombustion gas G, the higher the operation efficiency of the Rankinecycle apparatus 20. In an example, the highest temperature of combustiongas generated by a gas boiler is 1500° C., and the compositiontemperature of the organic working fluid is in a range of 150 to 300° C.

(Modification)

As illustrated in FIG. 2C, the temperature sensor 35 may be provided inthe most-upstream working fluid channel 25 a. In the modification, thetemperature sensor 35 is located in a range of L/2 upstream in the flowdirection of the working fluid from the downstream end 25 p of themost-upstream working fluid channel 25 a. Herein, L is defined as thewhole length of the most-upstream working fluid channel 25 a.Specifically, the temperature sensor 35 is attached to the connectingtube 29 constituting the most-upstream working fluid channel 25 a in theaforementioned range. It is possible to estimate the temperature of theworking fluid or the inner surface of the heat exchanger tube 28 at thedownstream end 25 p of the most-upstream working fluid channel 25 abased on the temperature detected at a position comparatively close tothe downstream end 25 p of the most-upstream working fluid channel 25 a.Accordingly, even when the temperature sensor 35 is provided at theposition illustrated in FIG. 2C, the control to adjust the temperatureof the working fluid in the evaporator 24 can be executed in a similarmanner to the case where the temperature sensor 35 is provided at theposition illustrated in FIG. 2B. The longer the distance between thedownstream end 25 p and the temperature sensor 35, the higher theestimation uncertainty of the temperature of the working fluid or thetemperature of the inner surface of the heat exchanger tube 28 at thedownstream end 25 p. Accordingly, the distance between the downstreamend 25 p and the temperature sensor 35 is preferably L/2 at the maximumas shown in the modification.

Hereinafter, a description is given of other embodiments of the CHPsystem. The same components of the other embodiments as those ofEmbodiment 1 are given the same reference numerals, and the descriptionthereof is omitted. Accordingly, the description of each embodiment isapplicable to the other embodiment as long as being technicallyconsistent with each other.

Embodiment 2

As illustrated in FIG. 5, a CHP system 200 of Embodiment 2 includes theboiler 10, a Rankine cycle apparatus 20B, the heat medium circuit 30,and the control circuit 50. The configuration of the CHP system 200 ofEmbodiment 2 is the same as that of the CHP system 100 of Embodiment 1except an evaporator 34 and a bypass circuit 43.

In the Rankine cycle apparatus 20B, the bypass circuit 43 is a circuitbypassing the expander 21. The bypass circuit 43 branches from the flowchannel connecting the outlet of the evaporator 34 and the inlet of theexpander 21 and joins the flow channel connecting the outlet of theexpander 21 and the inlet of the condenser 22 (or the inlet of thereheat unit). The bypass circuit 43 is provided with a flow-rate controlvalve 45. The opening of the flow-rate control valve 45 is controlled toadjust the circulation flow rate of the working fluid in the Rankinecycle apparatus 20B.

As illustrated in FIGS. 6A and 6B, the evaporator 34 includes a firsttemperature sensor 35 and a second temperature sensor 36. The firsttemperature sensor 35 is attached to the connecting tube 29 connected tothe downstream end 25 p of the most-upstream working fluid channel 25 aas described in Embodiment 1. In terms of the flow direction of theworking fluid, the second temperature sensor 36 is provided upstream ofthe upstream end 25 q of the most-upstream working fluid channel 25 a inthe working fluid channel 25. Specifically, the second temperaturesensor 36 is attached to the connecting tube 29 connected to theupstream end 25 q of the most-upstream working fluid channel 25 a. Thefirst and second temperature sensors 35 and 36 are both located outsideof the combustion chamber 12. Based on the output values of the firstand second temperature sensors 35 and 36, the temperature of the workingfluid in the evaporator 34 is adjusted. This can prevent the workingfluid from being excessively heated in the evaporator 34. Using the twotemperature sensors 35 and 36 enables more accurate temperature control.The first and second temperature sensors 35 and 36 are provided for twoconnecting tubes 29 adjacent to the downstream and upstream ends 25 pand 25 q of the most-upstream working fluid channel 25 a in particular.In this case, based on the difference (Th1−Th2) between temperature Th1detected by the first temperature sensor 35 and temperature Th2 detectedby the second temperature sensor 36, the state of the working fluid inthe working fluid channel 25 (the most-upstream working fluid channel 25a in particular) can be accurately known. The difference (Th1−Th2)represents the degree of excessive heating of the working fluid. Whenthe difference (Th1−Th2) is equal to zero, the working fluid is in thegas-liquid two-phase state all through the most-upstream working fluidchannel 25 a. When the difference (Th1−Th2) is larger than zero, theworking fluid is in the gas-phase state at the downstream end 25 p ofthe most-upstream working fluid channel 25 a.

The evaporator 34 includes partitions 40. One of the partitions 40 isprovided between the first temperature sensor 35 and the environmentaround the evaporator 34. Specifically, the partition 40 surrounds thefirst temperature sensor 35 and the connecting tube 29 to which thefirst temperature sensor 35 is attached. The partition 40 is a covermade of a resin plate, a metal plate, or the like, for example. Thepartition 40 reduces thermal influence of the environment around theevaporator 34 on the first temperature sensor 35. The other partition 40is the same as the aforementioned partition 40 and is provided betweenthe second temperature sensor 36 and the environment around theevaporator 34. With such a configuration, the temperature of the workingfluid flowing through the connecting tube 29 can be accurately detected.

Next, with reference to the flowchart of FIG. 7, a description is givenof a control which is executed by the control circuit 50 to adjust thetemperature of the working fluid in the evaporator 34. By executing thecontrol illustrated in FIG. 7, the temperature of the working fluid at aparticular position of the evaporator 34 approximates to the targettemperature. The particular position is the position where the firsttemperature sensor 35 is provided, for example.

First, temperatures Th1 and Th2 of the working fluid are detected withthe first and second temperature sensors 35 and 36, respectively (stepS11). Next, based on the two detected temperatures, it is determinedwhether the temperature of the working fluid in the evaporator 34 isexcessively high (step S12). Specifically, it is determined whether thedifference (Th1−Th2) between the two detected temperatures is equal toor less than threshold temperature previously set. When the difference(Th1−Th2) is more than the threshold temperature, the process to reducethe temperature of the working fluid is executed. Before execution ofthe process to reduce the temperature of the working fluid, the rotationspeed fp (operation frequency) of the expander 21 is detected (stepS13). The threshold temperature is in a range of 5 to 30° C., forexample.

Next, the temperature of the working fluid is reduced by increasing thecirculation flow rate of the working fluid. Specifically, the opening ofa flow rate valve 45 is increased (step S14). When the opening of theflow rate valve 45 is increased, the circulation flow rate of theworking fluid in the Rankine cycle apparatus 20 b is increased. When thecirculation flow rate of the working fluid is increased, the temperatureof the working fluid decreases in the evaporator 34. By controlling theopening of the flow-rate control valve 45, the temperature of theworking fluid in the evaporator 34 can be adjusted.

Next, it is determined whether the rotation speed fp of the expander 21is equal to or less than an upper limit (step S15). When the rotationspeed fp of the expander 21 is equal to or less than the upper limit,the rotation speed fp of the expander 21 is increased (step S16). Whenthe rotation speed fp of the expander 21 is increased, the circulationflow rate of the working fluid in the evaporator 34 increases, and thetemperature of the working fluid decreases. By controlling the rotationspeed fp of the expander 21, therefore, the temperature of the workingfluid can be adjusted in the evaporator 34. After the rotation speed fpof the expander 21 is increased, the temperatures Th1 and Th2 of theworking fluid are detected with the first and second temperature sensors35 and 36, respectively (step S17). It is then determined whether thedifference (Th1−Th2) between the two detected temperatures is equal toor less than the threshold temperature (step S18). The processes of thesteps S13 to S18 are repeated until the difference (Th1−Th2) becomesequal to or less than the threshold temperature.

The flow-rate control valve 45 may be an on-off valve which iscontrolled to only open and close states.

FIG. 8 is the same diagram as FIG. 4, illustrating a relationshipbetween the temperature of the working fluid in the evaporator 34 andthe temperature of the inner wall surfaces of the heat exchanger tubes.In Embodiment 2, the second temperature sensor 36 is attached to theconnecting tube 29 adjacent to the upstream end 25 q of themost-upstream working fluid channel 25 a. At this position, the workingfluid is in the gas-liquid two-phase state. By using the first andsecond temperature sensors 35 and 36, it is possible to know the statesof the working fluid including the degree of excessive heating of theworking fluid, pressure of the working fluid, and the like. It istherefore possible to make control after understanding the states of theworking fluid in detail.

As the number of temperature sensors increases, changes in temperatureof the working fluid in the evaporator 34 can be understood moreaccurately. However, increasing the number of temperature sensors meansincreasing the cost. Changes in the temperature of the working fluid inthe evaporator 34 can be predicted with the two temperature sensors 35and 36 as described in Embodiment 2. If the number of temperaturesensors is limited, it is possible to implementing safety control whilepreventing an increase in cost. With regard to the temperature of theworking fluid, the control to adjust the temperature of the workingfluid in the evaporator 34 may be executed by using not only thetemperature of the working fluid in the evaporator 34 but also thedetected value of a temperature sensor provided at another place in theRankine cycle apparatus 20B. Moreover, the control may be executed byusing pressure detected by a pressure sensor.

Embodiment 3

As illustrated in FIG. 9, a CHP system 300 of Embodiment 3 includes theboiler 10, a Rankine cycle apparatus 20C, the heat medium circuit 30,and the control circuit 50. The configuration of the CHP system 300 ofEmbodiment 3 is the same as that of the CHP system 100 of Embodiment 1except an evaporator 44 and an intake-air control fan 38 of the Rankinecycle apparatus 20C.

The intake-air control fan 38 is provided within the boiler 10.Specifically, the intake-air control fan 38 is provided in the flow pathfor supplying air to the combustor 14. The intake-air control fan 38plays a role of controlling the flow rate of air to be supplied to thecombustor 14.

As illustrated in FIGS. 10A and 10B, in Embodiment 3, one of the heatexchanger tubes 28 constituting the most-upstream working fluid channel25 a serves as the inlet of the evaporator 44 so that the working fluidentering the evaporator 44 first flows through the heat exchanger tubes28 constituting the most-upstream working fluid channel 25 a. The outletof the evaporator 44 is composed of one of the heat exchanger tubes 28constituting the intermediate working fluid channel 25 c. The workingfluid flows thorough the most-upstream working fluid channel 25 a, themost-downstream working fluid channel 25 b, and the intermediate workingfluid channel 25 c in this order.

The temperature sensor 35 is provided downstream of a downstream end 25r of the most-downstream working fluid channel 25 b in the flowdirection of the working fluid in the working fluid channel 25.Specifically, the temperature sensor 35 is attached to the connectingtube 29 connected to the downstream end 25 r of the most-downstreamworking fluid channel 25 b. When the temperature sensor 35 is located atsuch a position, the temperature of the working fluid at the downstreamend 25 r of the most-downstream working fluid channel 25 b can bedetected accurately.

Next, with reference to the flowchart of FIG. 11, a description is givenof a control which is executed by the control circuit 50 to adjust thetemperature of the working fluid in the evaporator 44. By executing thecontrol illustrated in FIG. 11, the temperature of the working fluid ata particular position in the evaporator 44 approximates to the targettemperature. The particular position is the position where thetemperature sensor 35 is provided, for example.

Steps S21 to 23 and S25 to S28 of the flowchart in FIG. 11 correspond tosteps S1 to S3 and S5 to S8 of the flowchart in FIG. 3, respectively,and the description of those steps is omitted.

In Embodiment 3, the temperature of the working fluid is reduced byreducing the temperature of the combustion gas G. Specifically, therotation speed fp of the intake-air control fan 38 is increased (stepS24). When the rotation speed fp of the intake-air control fan 38 isincreased, the amount of low-temperature air to be used in combustionincreases. The temperature of the combustion gas G generated bycombustion can be thereby reduced. In such a manner, the temperature ofthe working fluid in the evaporator 44 can be adjusted by controllingthe temperature of the combustion gas G. If the rotation speed fp of theintake-air control fan 38 is increased excessively, excess air supplyleads to a risk that the combustion cannot be maintained. Accordingly,it is desirable that the rotation speed fp of the intake-air control fan38 is maintained less than a predetermined upper limit.

FIG. 12 is the same diagram as FIG. 4, illustrating a relationshipbetween the temperature of the working fluid and the temperature of theinner wall surfaces of the heat exchanger tubes in the evaporator 44. InEmbodiment 3, the temperature of the inner wall surfaces of the heatexchanger tubes 28 is the highest at the outlet of the evaporator 44. InEmbodiment 3, the working fluid is maintained in the gas-liquidtwo-phase state all through the most-upstream working fluid channel 25a. Accordingly, thermal decomposition of the working fluid can beprevented. It is certainly not inhibited that the working fluid changesfrom the gas-liquid two-phase state to the gas-phase state in themost-upstream working fluid channel 25 a. Moreover, another temperaturesensor (a second temperature sensor) may be attached to the connectingtube 29 connected to the downstream end 25 p of the most-upstreamworking fluid channel 25 a.

In Embodiments 1 and 2, the inlet of the evaporator 24 or 34 is composedof one of the heat exchanger tubes 28 constituting the most-downstreamworking fluid channel 25 b. Accordingly, the combustion gas G exchangesheat with the working fluid of low temperature flowing through themost-downstream working fluid channel 25 b even in the latter part ofthe evaporator 24 or 34 as a heat exchanger. On the other hand,according to Embodiment 3, the temperature of the working fluid withwhich the combustion gas G exchanges heat in the latter part of theevaporator 44 as a heat exchanger depends on the state of the workingfluid in the most-downstream working fluid channel 25 b. The lower thetemperature of the working fluid, the larger the heat recovered from thecombustion gas. From the viewpoint of increasing the heat exchangeefficiency, it is desirable that the working fluid is in the gas-liquidtwo-phase state in a part or all of the most-downstream working fluidchannel 25 b.

Embodiment 4

As illustrated in FIG. 13, a CHP system 400 of Embodiment 4 includes theboiler 10, a Rankine cycle apparatus 20D, the heat medium circuit 30,and the control circuit 50. The configuration of the CHP system 400 ofEmbodiment 4 is the same as that of the CHP system 100 of Embodiment 1except an evaporator 54 and a combustor 14 of the Rankine cycleapparatus 20D.

As illustrated in FIGS. 13 and 14, in Embodiment 4, the combustor 14 iscylindrical. The evaporator 54 is a coil-type heat exchanger including acoil-type heat exchanger tube 28. The working fluid channel 25 iscomposed of plural stages in the flow direction of the combustion gas Gwhich exchanges heat with the working fluid. In Embodiment 4, theworking fluid channel 25 is composed of two stages. Specifically, theworking fluid channel 25 is composed of a most-upstream working fluidchannel 25 a and a most-downstream working fluid channel 25 b. Themost-upstream working fluid channel 25 a is formed at a positionrelatively close to the combustor 14, and the most-downstream workingfluid channel 25 b is formed at a position relatively far from thecombustor 14. The inlet of the evaporator 54 is composed of themost-upstream working fluid channel 25 a, and the outlet of theevaporator 54 is composed of the most-downstream working fluid channel25 b.

Between the gas-upstream and most-downstream working fluid channels 25 aand 25 b, a flow-path changing structure 39 is provided. The flow-pathchanging structure 39 is provided between the inlet to receive thecombustion gas G (the inlet of the evaporator 54) and the outlet todischarge the combustion gas G (the outlet of the evaporator 54) and isconfigured to interrupt the flow of the combustion gas G and change theflow direction of the combustion gas G. The flow direction of thecombustion gas G is different between in the inlet side and in theoutlet side. The space formed between the inner wall surface of thecombustion chamber 12 and the flow-path changing structure 39constitutes an exhaust path of the combustion gas G. In a similarmanner, the space between the inner wall surface of the combustionchamber 12 and the heat exchanger tubes 28 constitutes the exhaust pathof the combustion gas G. The flow-path changing structure 39 can be abaffle plate located between the gas-upstream and most-downstreamworking fluid channels 25 a and 25 b. The baffle plate has a circularprofile in a plan view, for example. The flow-path changing structure 39can determine the flow direction of the combustion gas G so that heatexchange is performed efficiently.

The working fluid channel 25 may be composed of one heat exchanger tube28 having a coil form or may be composed of plural exchanger tubes 28.Each of the gas-upstream and most-downstream working fluid channels 25 aand 25 b may be composed of one heat exchanger tube 28 having a coilform or may be composed of plural exchanger tubes 28. In Embodiment 4,the heat exchanger tubes 28 constituting the gas-upstream andmost-downstream working fluid channels 25 a and 25 b are coaxiallylocated and are connected to each other with a connecting tube 29. Atleast a part of the connecting tube 29 is placed outside of thecombustion chamber 12. The temperature sensor 35 and connecting tube 29may be surrounded by a heat insulator (Embodiment 1) or a partition(Embodiment 2).

The temperature sensor 35 is provided downstream of the downstream endof the most-upstream working fluid channel 25 a in the working fluidchannel 25. Specifically, the temperature sensor 35 is attached to theconnecting tube 29 in the outside of the combustion chamber 12.Similarly to Embodiments 1 to 3, the control to prevent thermaldecomposition of the working fluid can be executed based on the outputvalue of the temperature sensor 35. The temperature sensor 35 may beattached to one of the heat exchanger tubes 28 constituting the workingfluid channel 25 in the combustion chamber 12.

In a region near the combustor 14, the combustion gas G has hightemperature. Accordingly, it is desirable that the working fluid is inthe gas-liquid two-phase state or low-temperature gas phase in theregion near the combustor 14. On the other hand, in a region distantfrom the combustor 14, the combustion gas G has low temperature.Accordingly, the working fluid is allowed to be in the gas phase in theregion distant from the combustor 14. In this light, the state(temperature) of the working fluid in the evaporator 54 needs to beadjusted by using the temperature 35 provided downward of the downwardend of the most-upstream working fluid channel 25 a in a similar mannerto Embodiments 1 to 3.

In the CHP system 400 of Embodiment 4, the working fluid channel 25 isprovided around the combustor 14. Accordingly, the CHP system 400 ofEmbodiment 4 can be made compact as a whole compared with the CHPsystems of the other embodiments.

The technique disclosed in the specification is suitable for not onlyheat recovery systems that recover heat with working fluid and use therecovered heat but also cogeneration systems such as CHP systems. Thetechnique disclosed in the specification is particularly suitable forsystems frequently changing in electricity demand. Moreover, thetechnique disclosed in the specification is applicable to every systemincluding a process of heating the working fluid with high-temperaturefluid, such as high-temperature heat pumps.

What is claimed is:
 1. An evaporator which heats working fluid withhigh-temperature fluid to evaporate the working fluid, the evaporatorcomprising: a working fluid channel which is arranged to form aplurality of stages in a flow direction of the high-temperature fluidand through which the working fluid flows, and a first temperaturesensor which is provided for the working fluid channel, wherein theworking fluid channel is arranged to form a meander shape in theplurality of stages, and bent portions of the meander shape are exposedto outside of a housing of the evaporator, the plurality of stagesinclude a first stage located most upstream in the flow direction of thehigh-temperature fluid and a stage other than the first stage, theworking fluid channel allows the working fluid to flow out of theevaporator through an outlet of the working fluid channel which isincluded in the stage other than the first stage, the first temperaturesensor is provided in a part of the working fluid channel exposed to theoutside of the housing of the evaporator in a region between a firstpoint and a second point in the flow direction of the working fluid, thefirst point being at a distance of L/2 upstream in the flow direction ofthe working fluid from a downstream end of the part of the working fluidchannel forming the first stage where L is the whole length of the partof the working fluid channel forming the first stage, the second pointbeing at a distance of L/2 downstream in the flow direction of theworking fluid from the downstream end of the part of the working fluidchannel forming the first stage, an output value of the firsttemperature sensor is used to adjust temperature of the working fluid inthe evaporator, and the first stage is configured such that thehigh-temperature fluid flows by the working fluid in the first stageonly once in the flow direction of the high-temperature fluid.
 2. Theevaporator according to claim 1, wherein the plurality of stages includea second stage located next to the first stage downstream in the flowdirection of the high-temperature fluid, and the first temperaturesensor is provided between the first and second stages.
 3. Theevaporator according to claim 1, wherein the plurality of stages includea third stage which is located most downstream in the flow direction ofthe high-temperature fluid, and the working fluid channel allows theworking fluid to enter the evaporator through an inlet of the workingfluid channel which is included in the third stage.
 4. The evaporatoraccording to claim 1, wherein the working fluid channel includes aplurality of heat exchanger tubes provided within the housing of theevaporator and a plurality of connecting tubes corresponding to the bentportions of the meander shape.
 5. The evaporator according to claim 1,wherein at least the part of the working fluid channel forming the firststage is an inner grooved pipe.
 6. The evaporator according to claim 1,further comprising: a heat insulator surrounding the first temperaturesensor, wherein the heat insulator reduces thermal influence of anenvironment around the evaporator on the first temperature sensor. 7.The evaporator according to claim 1, further comprising: a partitionprovided between the first temperature sensor and an environment aroundthe evaporator, wherein the partition reduces thermal influence of theenvironment around the evaporator on the first temperature sensor. 8.The evaporator according to claim 1, further comprising: a secondtemperature sensor which is different from the first temperature sensorand is provided upstream of the first stage in a part of the workingfluid channel exposed to the outside of the housing of the evaporator,and output values of the first and second temperature sensors are usedto adjust the temperature of the working fluid in the evaporator.
 9. Theevaporator according to claim 1, wherein the first stage is configuredsuch that a plane intersects the part of the working fluid channelforming the first stage, the plane being perpendicular to the flowdirection of the high-temperature fluid.
 10. A Rankine cycle apparatus,comprising: a pump which pressurizes the working fluid; the evaporatoraccording to claim 1 which receives the working fluid discharged fromthe pump; an expander which expands the working fluid heated by theevaporator; a condenser which cools the working fluid discharged fromthe expander; and a control circuit.
 11. The Rankine cycle apparatusaccording to claim 10, wherein temperature of the high-temperature fluidis higher than decomposition temperature of the working fluid.
 12. TheRankine cycle apparatus according to claim 10, wherein the working fluidis an organic working fluid.
 13. A combined heat and power system,comprising: the Rankine cycle apparatus according to claim 10; and aheat medium circuit in which a heat medium flows as a low-temperatureheat source which cools the working fluid in the condenser of theRankine cycle apparatus.